<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style type="text/css">
    body
    {
        font-size: 60px;
    }
    .box1
    {
        width: 200px;
        height: 200px;
        background-color: red;
        position: absolute;
        /*
        对于开启了定位的元素，可以通过z-index属性来指定元素的层级
        z-index需要一个整数作为参数，值越大元素的层级越高
        元素的层级越高越优先显示
        如果元素的层级一样，则优先显示靠下的元素
        祖先元素的层级再高，也不会盖住后代元素
        */
        /*z-index: 3;*/
    }
    .box2
    {
        width: 200px;
        height: 200px;
        background-color:blue;
        position: absolute;
        top: 50px;
        left: 50px;
        /*z-index: 2;*/
    }
    .box3
    {
        width: 200px;
        height: 200px;
        background-color: green;
        position: absolute;
        top: 100px;
        left: 100px;
        /*z-index: 1;*/
    }
    .box4
    {
        width: 100px;
        height: 100px;
        background-color: yellow;
        position: absolute;
    }
    </style>
</head>
<body>
    <div class="box1">1</div>
    <div class="box2">2</div>
    <div class="box3">3
        <div class="box4"></div>
    </div>
</body>
</html>